Radiotherapy accelerators must have a means for monitoring the radiation delivered; modern machines use a transmission ionization chamber located in the beam path for this purpose. Typically, this chamber has signal electrodes divided into segments so as to measure the spatial distribution of the radiation beam. This invention is a means of measuring the total ionization current of the chamber independent of the measurement of the currents from each of the segments. Ionization occurring in volumes of the chamber to which the segments are unresponsive is included in this new means of current measurement. This provides assurance that the ionization chamber is providing an accurate measure of the radiation beam. Another approach would be to reduce the insensitive area of such signal electrodes.
In the dosimetry system of modern radiotherapy accelerators a segmented transmission ionization chamber is used to measure both total flux as well as the spatial distribution of the radiation beam.
Segmentation of the ion chambers is accomplished by dividing the signal, or low voltage electrode into a number of electrodes, as described in U.S. Pat. Nos. 4,131,799 and 3,852,610. These patents also describe appropriate construction techniques.
The total radiation flux is measured by electronically summing the signals originating from the various segments within the chamber. Spatial distribution measurements are obtained from the differential response of the various segments. Typical of such schemes is one described in U.S. Pat. No. 4,427,890. General descriptions of the schemes are given in ICRU Report 35 (International Commission on Radiation Units and Measurements, 1984) and in a review article by Karzmark (Medical Physics 11, pp. 123-124 (1984)).
Alternate schemes for monitoring the radiation field have included use of secondary emission foils in the beam path or use of a toroid pulse transformer around the beam. Both of these techniques work only for electron beams and are insensitive to photon or neutron beams.
A disadvantage of the prior art using segmented ionization chambers is that they have regions of insensitivity between the segments. This allows the accelerator to deliver radiation that may be undetected or inaccurately measured by the dosimetry system. Furthermore, the response of electronic circuits measuring the ionization current from various segments must be well matched if an accurate measure of total current is to be obtained by summing currents from these segments. Electronic circuits for measuring the currents from the chamber segments must be high quality because the currents are small and leakage currents are a problem.
In most accelerators the spatial information is also used to correctively tune the beam if an asymmetry is detected in the radiation flux. U.S. Pat. No. 3,838,284 covers this aspect of chamber design.
Shapes of the various segments are designed to provide information about various types of beam inhomogeneties which can result from a number of causes. This information can be used to drive servo systems which control the accelerator to correct such inhomogeneities and/or the information can operate interlock circuits to terminate radiation production if the spatial distribution of the beam is not correct. For example, U.S. Pat. No. 4,347,547 is a scheme whereby the beam intensity over a central electrode is compared to the sum of various outer segments to provide energy discrimination. Simultaneously the differences in beam intensity over the various outer electrodes provide information about the spatial symmetry in planes normal to the direction of the radiation beam.
A failure of one of the segments or its associated electronics may erroneously be interpreted by the system as a reduction and an asymmetry in the radiation flux. Should the control system try to correct this, a misapplication of the radiation beam may result. A means of measuring total ionization current independent of the segments is needed to verify the integrity of the segmented system.